<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>JS 代码示例</title>
</head>
<body>
    <script>
        var a;
        console.log(a);
        console.log(a + "10");
        console.log(a + 10);
    </script>
</body>
</html>